1. Field of the Invention
The present invention relates generally to the field of remelting metal scrap and specifically to the field of remelting lightweight aluminum scrap such as sheet metal scrap, machine shop turnings, and aluminum beverage cans.
2. Description of the Prior Art
A major concern of the secondary metal industry is the generation of oxides and gases which become included, entrained or dissolved into the molten metal during the remelting of scrap metal. These oxides are a source of concern in as much as the progeneration of oxides diminishes the proportion of remelted scrap metal which is ultimately marketable as end product. The percent of scrap rendered nonusable because of oxides generated during remelt is termed "melt loss."
Impurities form during remelting when the extremely reactive liquid metal surface area interfaces with reactive gases such as oxygen and hydrogen. A primary source of hydrogen and oxygen is the air and fuel gas combination used which is combusted to fire remelting furnaces. To combat impurities in the remelting of scrap metals, appropriate steps are taken to minimize the surface areas of reactive scrap metal exposed to reactive gases and means are employed to remove impurities by refining the molten metal with fluxes.
In the field of remelting scrap metals, the combative strategy used to prevent or inhibit the generation of oxides during remelting is generally as follows: Initially, a fluid bath of molten metal is formed in a furnace by remelting high-mass, low-surface area (heavy gauge) materials; then the bath is covered with a protective coating of flux and dross; then additional scrap is remelted by submerging such in the existing molten metal bath. Once the pool of molten metal has been established in the furnace it is brought to a preset level and drained off at a rate which is commensurate with additions of more scrap. The barrier coating which covers the bath is formed by fluxing the surface of the bath. The impurities, aided by the flux, float to the surface to form a crust or "dross" on top of the bath. The dross is in a solid or plastic state.
Directly beneath the dross, is a "skim" of semi-molten, semi-plastic metal which includes varying degrees of impurities. The skim and dross are normally removed either continuously or intermittently from the furnace to prevent large buildups, however it is considered beneficial to have some skim and dross on the top of the furnace to act as a barrier to prevent the additional combination of the molten metal underlying it with oxygen and other atmospheric gases. The skim and the dross that are removed from the furnace completely solidify and are either discarded or processed to reclaim entrapped metal.
The melting of scrap metal is an energy intensive process, and additional energy is required to convert and keep the metal scrap in the molten state. The form of energy used is heat, generated by an electric or a combustion source. The introduction of metal scrap to the heat generating areas is problematic in that great amounts of heat energy are lost both when relatively cold scrap metal is introduced to those heat generating areas and as those heat generating areas are exposed to the colder ambient air during the introduction of the scrap. In addition, impurities are generated when scrap is melted in intimate contact with fuel gases, combustion gases, and ambient air. As a general proposition metal melting furnaces have been redesigned in an attempt to specifically prevent heat loss when the heat generating area is exposed to ambient air, and furthermore to specifically prevent the fuel gases, combustion gases and ambient air from reacting with the melting scrap to form impurities.
One of the approaches to furnace modification has taken the form of fabricating an additional chamber, or "melting" chamber, exterior to the hearth, out of refractory materials. The refractory "walls" separate the burners from the melting chamber. Ports are built through the walls to permit molten metal flow between the melting chamber and the heating chamber. The ports are located below the fluid level normally set for the molten metal. The scrap is introduced into the melting chamber where it comes into contact with the molten metal. Using such an approach, the scrap is always secluded from the detrimental effects of the burners and the burners do not lose valuable heat energy to the ambient air.
In attempting to utilize such separate chamber designs for the remelting furnaces, it has been determined that natural convection of the molten metal by itself, through the ports in the walls between the separate chambers, is not adequate to transfer heat at a rate which is sufficient to both maintain the molten metal in the melting chamber in the molten state and to melt the solid phase scrap metal which is being added. Attempts have been made to utilize special refractory compositions which are designed to transfer the heat from the heating chamber into the melting chamber, with limited success, resulting in only relatively small quantities of scrap being successfully added without reducing the melt temperature below acceptable levels. The addition of special molten metal pumps is ultimately required to insure that the temperatures within the melting chambers are maintained high enough so that solidification does not begin to occur.
Such pumps, which are commercially available, are normally made from graphite or other refractory materials which resist deterioration. The addition of the molten metal circulation pump to the separate chamber melting furnace system described above enables the melting and the commercial application of this system to the remelting of relatively heavy gauge scrap. However, there has been an increasing market use of low-mass, high-surface area (light gauge) metals, resulting in increasingly greater quantities of light gauge scrap being available for recycling. In particular, there has been a great increase in the use of aluminum for beverage cans. With the remelting of light gauge scrap, additional problems are encountered and new approaches are needed to reduce melt losses to acceptable levels.
Molten metal is characterized by very high surface tension. Generally speaking, in metal remelting furnaces, heavy gauge scrap is dropped into the molten metal and, by gravity, rapidly sinks into the fluid where it melts. Due to the surface tension and the dross and skim on the top of molten metals, it is more difficult to include light gauge scrap into the fluid because of the fact that it tends to "float" for an extended period of time on the surface. Much of the light gauge scrap that is used is lost to oxidation and other chemical reactions as it begins to melt on the surface. It has been recognized that means are needed to quickly overcome the surface tension, thus ways are required to introduce light gauge scrap through the dross and skim into to melt beneath.
Initially, attempts were made to mechanically push the light gauge scrap under the molten metal surface. Also, attempts have been made to compress the light gauge scrap into large bundles followed by mechanically forcing the large bundle under the molten metal surface. Both of these methods have been unsuccessful due to excessive melt loss and low scrap metal melting and recovery rates. Beyond these initial attempts, improved methods have been developed to introduce the light gauge scrap beneath the molten metal surface in a separate melting chamber type furnace design.
An example of such an improved system is found in U.S. Pat. No. 4,286,985 wherein the molten metal is pumped from the heating chamber and directed into the upper portion of the melting chamber, thus providing both relatively hot molten metal at the upper portion of the molten pool in the melting chamber and creating a tumbling action in that chamber which has the purpose of swirling the scrap on the surface of the melt down into the melt itself.
Other designs have been introduced which include a variety of pump impeller arrangements designed to be placed directly into the melting chamber substantially below the surface level of the molten metal pool. Examples of such designs are found in U.S. Pat. Nos. 3,984,234, 4,128,415, and 4,322,245. In all of these designs, the scrap metal floating on the surface of the molten metal is drawn into the center of the molten pool by way of a fluid vortex created by the submerged impeller of the pump in that pool. The pump impellers of such systems serve the additional purpose of circulating the molten metal from the melting chamber into the heating chamber and from the heating chamber back into the melting chamber.
These systems have proved somewhat successful in that relatively more light gauge metal scrap can be drawn into the melt without exposure to contamination and oxidation as such material is melting. However these systems have not been entirely successful in practice because, when the impellers are driven at a sufficient speed to draw substantially all of the light gauge scrap rapidly down into the molten pool, a severe vortex is created. This severe vortex tends to also draw the surrounding atmospheric gases into the melt by suction. These gases combine readily with the molten metals to form high levels of impurities. This phenomenon is detailed in U.S. Pat. No. 4,322,245 column 4, lines 42-50. Thus there is a need for a system which will positively drive all of the light gauge scrap into the molten metal without inclusion of atmospheric gases and while also providing good molten metal circulation from the heating chamber to the melting chamber.